Mixed valence triiron complexes from the conjugation of [FeIFeI] and [FeII] complexes via intermolecular carbyne/alkyne coupling

Abstract

We present a new synthetic strategy for obtaining mixed-valence triiron complexes where the metal centers are bridged by a novel, highly functionalized hydrocarbyl ligand. The alkynyl–vinyliminium complexes [Fe₂Cp₂(CO)(μ-CO){μ-η¹:η³-C(X–CCH)CHCNMe₂}]CF₃SO₃ (X = 4-C₆H₄, [2a1]CF₃SO₃; X = (CH₂)₃, [2a2]CF₃SO₃) were synthesized in almost quantitative yields from the aminocarbyne precursor [Fe₂Cp₂(CO)₂(μ-CO){μ-CNMe₂}]CF₃SO₃, [1a]CF₃SO₃, and the di-alkynes HCC–X–CCH. Then, the ferracycle [Fe(Cp)(CO){C(NMe₂)CHC(4-C₆H₄CCH)C(O)}], 4a1, was produced in 47% yield from the cleavage of [2a1]CF₃SO₃ promoted by pyrrolidine. Subsequent reactions of the acetonitrile adducts [Fe₂Cp₂(CO)(μ-CO)(NCMe){μ-CNMe(R)}]CF₃SO₃ (R = Me, [1a^ACN]CF₃SO₃; R = Xyl, [1b^ACN]CF₃SO₃) ([Fe^IFe^I]) with 4a1 ([Fe^II]) at room temperature resulted in the formation of [Fe^IFe^IFe^II] complexes [Fe₂Cp₂(CO)(μ-CO){μ-η¹:η³-C(X–CCHC(NMe₂)FeCp(CO)CO)CHCNMe(R)}]CF₃SO₃ (R = Me, [5a1]CF₃SO₃; R = Xyl, [5b1]CF₃SO₃) in yields ranging from 56% to 64%. The new products were characterized by IR and multinuclear NMR spectroscopy, and the structures of [2a2]CF₃SO₃ and 4a1 were confirmed by single crystal X-ray diffraction. Cyclic voltammetry and spectroelectrochemical studies on [5a1]⁺ have revealed that reduction and oxidation events occur almost independently at the [Fe^IFe^I] and [Fe^II] units, respectively. This observation underscores a minimal electronic interaction between the two fragments within the triiron complex. Accordingly, DFT studies pointed out that the HOMO and LUMO orbitals are predominantly localized in the two distinct compartments of [5a1]⁺.